The ECU having two microcomputers together with a first power circuit that supplies power to one microcomputer and a second power circuit that supplies power to the other microcomputer is disclosed in a Japanese patent document, JP 2013-25471 A (patent document 1).
As disclosed in the patent document 1, the two microcomputers may be configured to monitor the one by using the other. One of the two microcomputers may simply be designated as a Microcontroller Unit (MCU) in the following, and the other one of the two microcomputers may be designated as a monitoring unit in such context.
Further, as for a power circuit for the microcomputer, a switching power source that has a transistor and steps down a voltage of an external power source by using a clock as well as a series power source that also has a transistor and steps down a voltage of the switching power source are known. Furthermore, the power source for supplying power to the microcomputer (i.e., a microcomputer power circuit) and the power source for supplying power to the monitoring unit (i.e., a monitor power circuit) may be provided with a clock from the same oscillator circuit.
In such configuration, when the output of the clock from the oscillator circuit stops, (i.e., when the oscillation of the oscillator circuit stops), a Pulse Width Modulation (PWM) signal having a preset duty cannot be generated, and the transistors in both of the microcomputer power circuit and the monitor power circuit will not be operable without the clock from the oscillator circuit.
For example, when the oscillation of the oscillator stops at a transistor OFF timing, the transistor stays in the OFF state, thereby causing a fall of the output voltage of the microcomputer power circuit and a fall of the output voltage of the monitor power circuit. Depending on the drive state of the microcomputer and the monitoring unit, as well as the charge state of the capacitor for the stability of the power source, the power source of the monitoring unit may fall prior to the fall of the power source of the microcomputer, which causes a no-monitoring period for the microcomputer. In such case, due to the abnormal calculation of the microcomputer, for example, an unintended control may be performed (e.g., a wrong torque instruction is output).